A trinket which derives its power from the wearer's mana. Thank you, Grandfather!
VII Limited Moon is Red XIII's ultimate weapon, obtained after talking to Bugenhagen with Red XIII in the party after the Raid on Midgar. It provides 93 Attack, 114 Atk%, 31 Magic, four double Materia slots with Nothing growth, and deals more damage the more MP Red XIII has according to the following formula:
This is then modified by the attack's power; for normal attacks, the multiplier is 1.0. Unlike most other damage modifiers, each character's ultimate weapon applies the damage modifier after all other modifiers, instead of modifying the attack strength of the weapon itself.
Assuming Power is 1, the weapon would have 4x its normal attack at full MP.